People living in places like New Delhi or Beijing may be at greater risk of developing chronic sinus problems due to high levels of air pollution in these cities, say researchers.

In the study, published in the American Journal of Respiratory Cell and Molecular Biology, the researchers found evidence that breathing in the dirty air directly causes a breakdown in the integrity of the sinus and nasal air passages in mice. "In the US, regulations have kept a lot of air pollution in check, but in places like New Delhi, Cairo or Beijing, where people heat their houses with wood-burning stoves, and factories release pollutants into the air, our study suggests people are at higher risk of developing chronic sinus problems," said Murray Ramanathan, Associate Professor at the Johns Hopkins University School of Medicine.

Researchers have long known that smog, ash and other particulates from industrial smokestacks and other sources that pollute air quality exacerbate and raise rates of asthma symptoms, but had little evidence of similar damage from those pollutants to the upper respiratory system.

To see how pollution may directly affect the biology of the upper airways, the researchers exposed mice to either filtered air or polluted air. The aerosolised particles, although concentrated, were 30 to 60 percent lower than the average concentrations of particles of a similar size in cities like New Delhi, Cairo and Beijing, the researchers said.

Nineteen mice breathed in filtered air, and 19 breathed polluted air for six hours per day, five days a week for 16 weeks. The researchers used water to flush out the noses and sinuses of the mice and then looked at the inflammatory and other cells in the flushed-out fluid under a microscope.

They saw many more white blood cells that signal inflammation, including macrophages, neutrophils and eosinophils, in the mice that breathed in the polluted air compared with those that breathed in filtered air. When the researchers examined layers of cells along the nasal passages and sinuses under a microscope, they found that the surface layer - or epithelium - was, notably, 30 to 40 percent thicker in mice that breathed in the polluted air than in those that breathed filtered air.

A thicker epithelium is another sign of inflammation in humans and other animals, Ramanathan said. "We've identified a lot of evidence that breathing in the dirty air directly causes a breakdown in the integrity of the sinus and nasal air passages in mice," Ramanathan said.  "Keeping this barrier intact is essential for protecting the cells in the tissues from irritation or infection from other sources, including pollen or germs," he explained.